Magnetic resonance imaging method and apparatus

ABSTRACT

An MRI apparatus is provided for obtaining an image of a wide area using a multiple-coil, where a plural number of small receiving coils are arranged such that adjacent coils overlap spatially. An image is picked up while thinning a part of the measured data, such that the low spatial frequency region of the k space is dense and the high spatial frequency region thereof is sparse. The substantial sensitivity distribution of each small receiving coil is determined using data related to the low spatial frequency region, and an image is composed using the sensitivity distribution and the measured data to produce a high resolution image having no aliasing artifact in a short time.

FIELD OF THE INVENTION

The present invention relates to a nuclear magnetic resonance imaging(MRI) method and to apparatus for continuously measuring a nuclearmagnetic resonance (hereinafter this is referred to as “NMR”) signalobtained from hydrogen or phosphorus in an object to be examined, andfor imaging the density distribution and relaxation time distribution orthe like of a nuclear particle in the object.

BACKGROUND OF THE INVENTION

As a receiving coil to detect an NMR signal generated from an object inan MRI apparatus, a high sensitivity coil called a “multiple RF coil” or“phased array coil” has been used in recent years (Japanese Patent laidopen No. 2-500175). A multiple RF coil is a coil dedicated to signalreception composed of an array of small type RF coils having arelatively high sensitivity, which is capable of receiving a signal in awide field of view with high sensitivity. Various types of multiple RFcoils have been proposed, including a static magnetic field type or adetecting part. In operation, the signals received with the respectiveunit coils of this multiple RF coil are combined to produce an image ofthe object to be imaged.

On the other hand, a method to shorten the imaging time by thinning outdata in the phase encoding direction and using multiple coils has beenproposed, for example, by [4] Daniel K Sodickson and Warren J Manning inan article entitled “Simultaneous obtainment of spatialharmonics(SMASH): fast imaging with radio frequency coil arrays”, inMagnetic Resonance in Medicine, Vol. 38, pages 591-603, (1997), and by[5] J.Wang and A.Reykowski in an article entitled “A SMASH/SENSE relatedmethod using ratios of array coil profiles”, in ISMRM 99. This kind oftechnology is referred to as a space encoding method or parallel MRI. Analiasing artifact in thinning phase encoded data is removed using thespatial difference between respective sensitivity distributions of amultiple RF coil. However, for the removal of this aliasing artifact, ahighly accurate calculation using a highly accurate sensitivitydistribution of the RF coil is necessary. This operation is performed ina measurement space (k space) in the method described in the aboveliterature [4]. And, in the method described in the literature [5], theoperation is performed in real space after Fourier transformation.

Generally, a sensitivity distribution of the multiple RF coil can becalculated from each of the received RF signals. More specifically, amethod that images a phantom having a uniform density previously andregards the spatial shading of an image as a sensitivity distribution ofthe multiple RF coil, and a method that calculates it by use of a lowfrequency filter in connection with an image measured separately fromthe object, are known. However, these traditional methods involve thefollowing problems.

That is, in the traditional methods, the process of calculating thesensitivity distribution of said RF coil is performed before imaging.Thus, for example, when imaging a part of an object, such as the abdomenof a patient, which changes its shape temporally with breathing, theshape of the object examined is likely to be different between the timethe sensitivity distribution is calculated and the time the imaging isperformed.

Also, the spatial arrangement of the RF coil fitted to the object isliable to change in accordance with the movement. In addition, in caseof imaging a patient during a surgical operation, the position of the RFcoil can change with time.

Also, in case there is a demand to display these images in real time,the traditional method of using sensitivity data of the coils obtainedpreviously can lead to an error, with a result that the quality of theimage can be deteriorated.

Furthermore, measuring the sensitivity distribution of an RF coil priorto imaging extends the total imaging time, and so it creates a problemin that the effect of short-time imaging, which is a main characteristicof this technology, is deteriorated.

The present invention has been developed for the purpose of solving theabove-mentioned problems.

SUMMARY OF THE INVENTION

To solve the above-mentioned problems, the present invention provides amagnetic resonance imaging method which involves the steps of:

(a) applying an RF pulse, a slice encoded gradient magnetic field, aphase encoded gradient magnetic fields, and a readout gradient magneticfield to the object to be examined, that is placed in a uniform staticmagnetic field, in accordance with a predetermined pulse sequence, andexecuting this pulse sequence repeatedly;

(b) detecting an NMR signal generated from the object to be examined byexecuting said step at each small type: RF coil composing a multiplecoil, and storing the NMR signals in connection with the k spaceseparately;

(c) performing a process of calculation of the sensitivity distributionof each coil using data only of a low spatial frequency region(hereinafter this is referred to as a “low region”) at each k space; and

(d) performing the image composing process using the sensitivitydistribution of a coil calculated in said step (c) and the measured datastored in connection with said k space.

The magnetic resonance imaging method of the present invention alsoinvolves steps of:

(a) preparing a k space having a predetermined matrix size for which tomemorize NMR signals detected from the object to be examined;

(b) executing a pulse sequence for NMR imaging of the object to beexamined, which has been placed in a uniform static magnetic field;

(c) memorizing NMR signals obtained by executing said pulse sequenceinto said k space;

(d) calculating the sensitivity distribution of a plural number of smalltype signal receiving coils forming a multiple coil by using one part ofthe NMR signal for imaging, which is memorized in connection with said kspace; and

(e) composing the image by using the sensitivity distribution of thesmall type signal receiving coil calculated above and the NMR signalmemorized in connection with said k space.

The magnetic resonance imaging method of the present invention is amethod of imaging an object to be examined by using a multiple coilwithout the presence of an aliasing artifact, and involves the steps of:

(a) preparing a k space having predetermined matrix for which size tomemorize NMR signals detected from the object to be examined with anumber corresponding to small type signal receiving coil composing amultiple coil;

(b) executing a pulse sequence for the NMR imaging of the object to beexamined, which is placed in uniform static magnetic field; the data inlow region in the phase encoded direction being measured fine in the kspace, and the data in high spatial frequency region (hereinafterreferred to as a “high region”) being measured roughly when this pulsesequence is executed;

(c) memorizing the measured data for imaging obtained by executing saidpulse sequence in connection with the k space corresponding to a smalltype coil;

(d) calculating the sensitivity distribution of a plural number of smalltype signal receiving coils forming a multiple coil by using a part ofthe data measured for imaging and memorized in connection with said kspace; and

(e) composing an image of all fields of view of the multiple coil byusing said calculated sensitivity distribution of said small type signalreceiving coil and measured data memorized in connection with said kspace.

The magnetic resonance imaging method of the present invention is amethod for continuously imaging an object to be examined by using amultiple coil, comprising:

(a) preparing a k space having a predetermined matrix size to memorizeNMR signal detected from the object to be examined;

(b) executing a pulse sequence for imaging a first NMR image of theobject to be examined, which has been placed in a uniform staticmagnetic field;

(c) memorizing NMR signals obtained by executing said pulse sequence inconnection with said k space;

(d) calculating the sensitivity distribution of a plural number of smalltype signal receiving coil which form a multiple coil by using a part ofthe NMR signals for imaging and memorizing it in connection with said kspace;

(e) composing an image by using said calculated sensitivity distributionof a small type signal receiving coil and said NMR signal memorized inconnection with said k space; and

(f) executing a pulse sequence for imaging after the second image andcomposing an image by applying said memorized sensitivity distributionof said small type signal receiving coil to the obtained NMR signal.

Problems addressed by the present invention also can be solved byprovision of a magnetic resonance imaging apparatus. In this regard, amagnetic resonance imaging apparatus of the present invention comprises:

a magnet for generating a uniform static magnetic field within the spaceaccommodating the object to be examined;

a multiple coil comprised of a plural number of small type coils fordetecting NMR signals generated from said object to be examined, saidplural number of small type coils being arrayed to overlap a part ofadjacent coils;

means for applying a high frequency magnetic field, a slice encodedgradient magnetic field, a phase encoded gradient magnetic field and areadout gradient magnetic field to image said object to be examined,where the phase encoding direction is directed in the direction ofarrangement of said multiple coil;

means for controlling said magnetic field applying means, includingmeans for modifying a step change in the application amount of saidphase encoded gradient magnetic field in its high region relative to itslow region;

measured data memorizing means for memorizing NMR signals detected insaid multiple coil corresponding to each small type coil;

means for calculating the sensitivity distribution of each small typecoil by using data of a low region in said phase encoded direction inevery NMR signal detected by each small coil; and

means for composing an image from said sensitivity distribution and saiddata memorized in said measured data memorizing means.

The magnetic resonance imaging apparatus of the present inventioncomprises:

a magnet for generating uniform static magnetic field within a spaceaccommodating the object to be examined;

a multiple coil comprised of a plural number of small type coils, whichare arranged to overlap with each other in a part of the adjacent coils,to detect NMR signals generated from said object to be examined;

means for applying a high frequency magnetic field, a slice encodedgradient magnetic field, a phase encoded magnetic field and a readoutgradient magnetic field in accordance with a predetermined pulsesequence to image said object to be examined;

a k space for memorizing NMR signals detected by said multiple coilcorresponding to each small type coil;

means for controlling said magnetic field applying means, includingmeans for modifying a step change in the high region relative to the lowregion of the k space memorized measured data;

means for calculating the sensitivity distribution of each small typecoil by using the data of the low region in said k space in every NMRsignal detected by a small type coil; and

means for composing an image from the sensitivity distribution and thedata memorized in said memorizing means.

The magnetic resonance imaging apparatus of the present inventioncomprises:

means for executing a measurement repeatedly for imaging a predeterminedslice of an object to be examined in an imaging unit;

means for calculating the sensitivity distribution of a multiple coil byusing at least one part of measured data obtained for one image andmemorizing it;

means for composing an image of all fields of view of the multiple coilby using said calculated sensitivity distribution; and

means for applying the memorized sensitivity distribution withoutrenewal to said image composition based on measured data aftercalculating the sensitivity distribution.

According to the present invention, because the sensitivity distributionof a small type RF coil is calculated from data obtained in ameasurement for imaging, there is no difference in time betweengeneration of the sensitivity distribution data and the measured data.Thus, even in MRI measurement in which the condition changes from timeto time, it is easy to maintain stability. And, even in a measurementthat demands real-time measurement, an error is not invited and theimage quality is not deteriorated.

And, according to the present invention, since filtering is performed soas to connect the data of low region to zero smoothly in calculating thesensitivity distribution using data of the low region in the k space, asensitivity distribution with no influence of an aliasing artifact canbe calculated.

Also, since the sensitivity distribution of the RF coil is measuredprior to imaging using measured data for imaging, without measuring thesensitivity distribution of the RF coil, the total imaging time does nothave to be extended, and the effect of imaging in a short time does notto be degraded.

According to the present invention, in an embodiment for measuring animage continuously, the collection of measured data and imagereconstruction following it are performed continuously, and control todisplay a plural number of images that last in terms of time can beperformed. Also, the image reconstruction means uses the sensitivitydistribution calculated from one measured data for reconstructing aplural number of images. Thus, in dynamic imaging in parallel MRI, thetime for image reconstruction can be shortened and real-time measurementcan be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an MRI apparatus which to the presentinvention is applied;

FIG. 2 is a block diagram showing the main part of the MRI device shownin FIG. 1;

FIG. 3 is a waveform timing diagram showing one embodiment of a pulsesequence applied to an MRI apparatus of the present invention;

FIG. 4 is a diagram showing one example of a k space data array ofmeasured data based on the pulse sequence shown in FIG. 3;

FIG. 5 is a schematic block diagram showing one embodiment of a signalprocessing unit in MRI apparatus of the present invention;

FIG. 6 is a diagram illustrating a sensitivity distribution calculationcarried out in accordance with the present invention;

FIGS. 7(a) and 7(b) are diagrams which show the correspondence betweenthe k space and the filter profile of a filter used in the filterprocess in accordance with the present invention;

FIG. 8(a) is a diagram illustrating the imaging of an object using twocoils, and FIGS. 8(b) and 8(c) are diagrams which show the imagesproduced by the two coils, respectively, in accordance with the presentinvention;

FIG. 9 is a diagram showing an embodiment of a signal composition methodof the present invention;

FIG. 10 is a diagram showing one embodiment of a continuous imagingprocess using the MRI apparatus of the present invention;

FIG. 11 is a timing diagram showing another embodiment of a pulsesequence for use in the MRI apparatus of the present invention;

FIG. 12 is a diagram showing another example of a k space data array ofmeasured data according to the present invention;

FIGS. 13(a) to 13(c) are diagrams illustrating another embodiment of asensitivity distribution calculation according to the present invention.

BEST MODE FOR CARRYING OUT OF THE INVENTION

Hereinafter, various embodiments of the present invention will bedescribed with reference to the drawings. FIG. 1 is a view showing thecomposition of a typical MRI apparatus to which the present invention isapplied. This MRI apparatus comprises a magnet 102 for generating astatic magnetic field around the object 101 to be examined, a gradientmagnetic field coil 103 for generating a gradient magnetic field in saidspace, an RF coil 104 for generating a high frequency magnetic field inthis region, a signal receiving RF coil 105 for detecting an NMR signalgenerated from the object 101 to be examined, and a bed 112 forsupporting the object 101 to be examined.

Gradient magnetic field coil 103 is composed of one pair of coils forgenerating a gradient magnetic field in three directions X, Y, Z,perpendicular to each other, and each gradient magnetic field coilgenerates a gradient magnetic field in accordance with the signal outputfrom a gradient magnetic field power supply 109. By changing the methodof application of the gradient magnetic field, an arbitrary slice of theobject 101 can be selected, and position information can be given in theMR signal. A gradient magnetic field which applies position informationto the MR signal is called a phase encoded gradient magnetic field, or areadout gradient magnetic field, by which a measuring space (k space),where measured data is disposed, is provided.

The RF coil 104 generates a high frequency magnetic field in accordancewith a signal received from the RF signal transmitting unit 110. Thefrequency of this high frequency magnetic field is tuned with theresonant frequency of the nuclear spin that is an imaging target. Animaging target of the ordinary MRI is a proton, which is the mainconstituent material of the object. The signal received by the RF coil105 is detected in signal detection unit 106, processed in the signalprocessing unit 107, and is also converted to an image signal throughcalculation.

As seen in FIG. 2, the RF coil 104 is a multiple coil arrangement 201formed of a number of small type RF coils 202(in the figure, forexample, four coils are provided). As each small type RF coil 202, forexample, a rectangular surface coil of 15 cm on one side can be used,and those adjacent rectangular coils are disposed to overlap one anotherin the phase encoded direction of the measurement. They have detectingsensitivity regions divided relative to each other.

Each small type RF coil 202 is connected to a respective preamplifier203, and the output from each coil is amplified in the respectivepreamplifier 203. Each preamplifier 203 is respectively connected inparallel to an AD convert-quadrature detect circuit 204, so as to formthe signal detection unit 106. Signals detected in the respective ADconvert-quadrature detect circuits 204 are transferred to the signalprocessing unit 107, which operates to perform Fourier transformation,filtering, composition calculation or the like for each coil. An MRIapparatus having a signal detecting part formed of a plural number ofsystems connected to multiple coils and each RF coil for composing itmay be called a parallel MRI system. The process performed at signalprocessing unit 107 is previously programmed to build in a controllingpart or memory device, which will be described later.

In FIG. 1, the gradient magnetic field power supply 109, the RFtransmitting unit 110, the signal detection unit part 106 are controlledwith their movement by control unit part 111, in accordance with a timechart generally referred to as a pulse sequence. In the MRI apparatus ofthe present invention, the control unit 111 controls the application ofphase encoding in one part of a region of the k space so as to be rough.

Next, an imaging method using the above-described MRI apparatus will bedescribed with reference to FIG. 3-FIG. 9.

FIG. 3 is a waveform timing diagram showing one example of a pulsesequence that is adopted for continuous imaging. This pulse sequenceemploys a gradient echo (GrE) method. After applying an RF pulse 301together with a slice encoded gradient magnetic field pulse 302 andgenerating transverse magnetization by exciting the nuclear spin in aspecified region of the object to be examined a phase encoded gradientmagnetic field pulse 303 is applied, and then a readout gradientmagnetic field pulse 304 is applied and echo signal 305 is measured. Theecho time TE, which is the time period between the time of applicationof the RF pulse and the point in time that the echo signal reaches itspeak value during measuring of the echo signal 305, is a parameter usedto define the contrast of an image, and it is set in advance consideringthe organization or the like of the target.

By repeating this sequence a plural number of times while changing theapplication amount of the phase encoded gradient magnetic field (this isthe integrated value of the gradient magnetic field intensity and theapplication time), obtained data is disposed to the k space.

FIG. 4 is a view showing an example of the k space data arrangement (ktrajectory) of measured data that is measured with repetition of saidsequence, and it is filled up with a signal obtained by performing onepulse sequence per one line of the k space in the transverse direction(kx direction). Also, the value of ky in the k space is defined by theapplication amount of the phase encoded gradient magnetic field given tothe measured NMR signal. In a typical GrE sequence, the step of thephase encoded amount has an equal interval at each measurement. But, inthe sequence of this embodiment, the phase encoded step in the region1(402) differs from that in the region 2(403) on the k space. Forexample, in region 1(402), occupying a low region (low spatial frequencyregion) part in the phase encoded direction, the signal is closelyspaced in the phase encoded (ky) direction. On the other hand, in region2(402), occupying a high region (high spatial frequency region) part,the signal obtained is more widely spaced in the phase encodeddirection. For example, in the data region occupying 10% up and downfrom the center of the k space (20% of the center part in all), thephase encoded amount is increased one step at a time. But, when itexceeds more than 10% each up and down, the phase encode amount isincreased two or four steps at a time. As a result, the whole imagingtime can be shortened, because the number of executing pulse sequencesis reduced for a thinned number.

The ratio in the whole k space occupied by region 1(402) (about 20% inthe embodiment of the present invention) can be set appropriately byconsidering both the coil sensitivity and the shortening of themeasuring time. When the coil sensitivity is sharp, it is preferable toincrease the ratio. On the other hand, it is preferable to reduce theratio from the viewpoint of shortening the measuring time. Dataarrangement in the k space is typically in the form of a 256×256 matrixin the directions of kx and ky. In region 1(402), data is collected inthis density. In region 2(403), data is sampled two to four timesroughly of the typical data arrangement in the direction of phaseencoding. This means that a data measurement is performed in region1(402) with a matrix by which the field of view (FOV) can be displayedin 256 pixels in the phase encoded direction, and measurement isperformed in region 2 to produce a FOV of ½ to ¼.

The measured data is transferred to the signal processing unit 107 perreceiving coil, and measured data per receiving coil is taken in adifferent k space. Then, as shown in FIG. 5, the sensitivitydistribution of each receiving coil is calculated on the basis of thedata of each receiving coil (503), and a composing process to form animage is performed with the sensitivity distribution of each receivingcoil and a signal from each receiving coil (504). That is, by usingsignal e_(n) (kx, ky) from each coil, a sensitivity distribution imageW_(n) (x, y) of each coil is calculated. Here, n given to e_(n) andW_(n) refers to the coil number. In this embodiment, n is 1,2,3 or 4.Also, (kx,ky) represents coordinates of the k space and (x, y)represents a position in real space. By using the sensitivitydistribution image and signal, the entire image S(x, y) 505 is composed.The sensitivity distribution calculation 503 and composing process 504will be described in more detail.

FIG. 6 is a view showing the procedure used to calculate the sensitivitydistribution of an RF coil from a signal e_(n)(kx,ky) 501 of eachreceiving coil. At first, a low pass filter (LPF) 601 is used to retainonly data of region 1 in the signal e_(n)(kx,ky) in the phase encodeddirection. This filtering process is performed as a pre-processing toremove the aliasing artifact that is generated in the phase encodeddirection by a difference in the FOV in the measuring region 1(402) andthe region 2(403), when Fourier transformation is performed over thewhole k space (401) including region 1(402) and region 2(403). To removethe aliasing artifact, a one-dimensional filter can be used, but atwo-dimensional filter is preferable for removing the minute structureof a living body. For example, a Gaussian type, a humming type, and ahanning type are suitable for 2 dimensional filters. As a more highlyprecise filtering method, it is possible to employ a method using aflying window in the image space.

FIGS. 7(a) and 7(b) show the correspondence between the k space and thefilter profile of a filter used in the filtering process. When filteringis performed in the k space with a filter having such a filter profile,all of the data of region 2(403), as shown in FIG. 7(b), is zero, andthe boundary between region 1(402) and region 2(403) can be connectedsmoothly. It is effective to smoothly connect the boundary between theregion 1(402) and the region 2(403), rather than to have it changesharply, for preventing generation of an aliasing artifact

Next, measured data arranged in region in the k space corresponding toeach coil is subjected to two-dimensional Fourier transformation (FT)602. But, before that, the matrix of each k space to be subjected totwo-dimensional Fourier transformation is made to have the density formeasurement of region 1, and region 2 in each k space is filled withzero. And, with the two-dimensional Fourier transformation beingperformed at each k space, an image composed only of a low frequencycomponent in the signal measured at each coil can be taken out,respectively. It is known that these low frequency images are regardedas the sensitivity distribution of each RF coil. In this embodiment,this is defined to be the sensitivity distribution W_(n)(x,y)

Incidentally, in multi-slice imaging, the sensitivity distribution ofthe coils needs to be calculated at each slice of each coil. And,because the signal obtained at each coil becomes a function of thethree-dimensional measuring space (kx,ky,kz), in case of threedimensional imaging, it is possible to calculate a three-dimensionalsensitivity distribution W_(n)(x, y, z) of the coil by expanding theFourier transformation to three dimensions by using a three-dimensionalfilter as the low pass filter. Next, the operation 504 for obtaining acomposed image by using this sensitivity distribution W_(n)(x,y) will bedescribed. This composing calculation involves a method performed in theimaging space (real space) and a method performed in the measuringspace. Both of them can be adopted.

FIGS. 8(a), 8(b) and 8(c) conceptionally illustrate a method forperforming composition of an image in the imaging space. To simplify theexplanation, a case of imaging the object by using a multiple coilcomposed of two small type coils 801, 802 will be considered.

In this case, coil 801 has a FOV1, and coil 802 has a FOV2. Thesensitivity distribution of each coil overlaps that of the other coil atleast at one part. Measurement of the NMR signal is, for example,performed with the GrE sequence shown in FIG. 3, and echo signalsmeasured at coil 801 or coil 802 are arranged respectively in the kspace as shown in FIG. 4. As explained above, performing two-dimensionalFourier transformation in two k spaces that take in a signal separatelyfrom two coils, as the data is collected roughly (by thinning out) inthe phase encoded direction in region 2 of the k space, as shown in FIG.4, then, an image coming from coil 1(801) is reconstructed as shown inFIG. 8(b), and an image coming from coil 2(802) is reconstructed asshown in FIG. 8(c). And at each image, an aliasing artifact 804 isgenerated in phase encoded direction on image 803. These artifacts havea low brightness because they are signals from a low sensitivity regionat each coil.

The aliasing artifact 804 can be removed by multiplying the artifactsensitivity distribution of each small type coil, so as to obtain animage having no overlapping on the images due to the aliasing artifact,as shown in FIG. 8(a). This method has been referred to in the section“SENSE” of the above-disclosed publication by said J. Wang et al.;therefore, a detailed explanation thereof will be omitted.

In the method of composing a signal in the measuring space, the composedsensitivity distribution obtained by the composed sensitivitydistribution of each small type RF coil with appropriate weights isdefined to have a desired spatial frequency in accordance with thenumber of small type RF coils which form the multiple coil. Then, thedata lacking in the imaging space is made up.

For example, it is assumed that the composed sensitivity distributionWcomp(x,y) of the form exp(i·mΔky·y), where m is an integer, can beobtained by the composed sensitivity distribution W_(n)(x,y) of a smalltype RF coil with appropriate weight Cn.

Wcomp(x,y)=ΣCjWj(x,y)=exp(imΔky·y)  (1)

Signal S(kx,ky) composed at this point is shown in the next equation.

S(kx,ky)=∫∫dxdyWcomp(x,y)ρ(x,y)exp{−ikxx−ikyy}  (2)

=∫∫dxdyp(x,y)exp{−ikxx−i(ky-mΔky)y}

=ρ{circumflex over ( )} (kx, ky-mΔky)

In the equation (2), ρ represents magnetization density, and {circumflexover ( )} represents two-dimensional Fourier transformation. As isunderstood from the equation (2), ρ{circumflex over ( )} (kx,ky−mΔky)can be calculated from ρ{circumflex over ( )}(kx,ky) by using thecomposed sensitivity distribution Wcomp(x,y). This means that data inthe interval between the data that is roughly arranged in the phaseencoded direction of k space can be filled.

FIG. 9 is the view showing this method conceptionally. As shown in thefigure, at first, the sensitivity distribution can be calculated withzero filling (making data to be zero in the region 2 of the k space,except region 1 with the density of region 1) and two-dimensionalFourier transformation processing (2DFT) of measured data in the region1(402) extracted with LPF processing. Next, for region 2(403), wheremeasured data is arranged roughly by thinning out, lacking data(corresponding to the dotted line in the enlarged figure) is formed frommeasured data (corresponding to the solid line in the enlarged figure)from said calculation. The number of new data (number of dotted lines)formed by said process depends on the number of small type RF coils.When the number of small type RF coils is four, four arrangements of newdata can be formed. In the example shown in the figure, the thinning-outrate is four and the number of data being formed is three.

Thus, the data after being supplemented to obtain new data is the sameas the data measured without thinning out the entire region of k space,and an image without an aliasing artifact can be obtained by performingtwo-dimensional Fourier transformation to this data. Incidentally, sincethis method has been referred to in the above-mentioned document “SMASH”written by Daniel K Sodickson et al, a detailed explanation thereof willbe omitted.

According to the embodiment of the present invention, imaging isperformed while shortening the time by thinning out a part of the phaseencoding, and an RF coil sensitivity distribution is obtained with noinfluence of aliasing using data in a precise part of measured dataobtained for the imaging. Then, the time difference between themeasurement for calculating the sensitivity distribution and themeasurement for imaging can be eliminated, and the operation errorfollowing the time difference can be eliminated.

In addition, according to the embodiment of the present invention, byobtaining especially thick data of a low phase encoded component,including main image information in the measured data, the S/N ratio ofthe images are hard to be deteriorated and extremely effective imagescan be produced in a diagnosis of a clinical application. Moreover, byadding the process of performing Fourier transformation thickly to thetwo-dimensional image “(256×256 pixels)” without an aliasing artifactbeing obtained by said method again in the phase encoded direction, andby replacing the data estimated in the high region of the phase encodeddirection from the data obtained (appearing alternately on the phaseencoding) with measured data already obtained thickly (Fouriertransformed already in the readout direction), or adding and averagingthe estimated data and actual measured data in a hybrid space, theobtained data in region 1 can be utilized not only for removing analiasing artifact, but also 100% as a data element for imagereconstruction. Therefore, a final image with high S/N ratio can beobtained.

Furthermore, according to the embodiment of the present invention, it isnot necessary to perform a previous measurement for calculating thesensitivity distribution of a coil. Thus, not only is a time-shorteningeffect in measurement for imaging achieved, but also a shortening of themeasuring time as a whole can be achieved. In case of continuousimaging, it is preferable to attain both the effect of said high S/Nratio and the time-shortening effect of imaging. FIG. 10 shows anembodiment in which the present invention is applied to a dynamicimaging for obtaining a continuous image in real time by repeatingimaging in a time series. In the dynamic imaging according to thisembodiment, the signal obtained respectively at each small type RF coilof the multiple coil is composed, and image display/transfer is repeatedcontinuously, whereby continuous image (image number1,2,3 . . . 1000) isobtained in time series. In the first image obtained, each step of thesignal obtainment 1001, sensitivity distribution calculation 1002, andsignal composing 1003 are performed similarly to the steps 502, 503, 504shown in FIG. 5, and an image in which the aliasing artifact is removedis reconstructed by using the sensitivity distribution calculatedaccording to the sensitivity distribution calculation 1002. This imageis displayed on the display of the MRI apparatus, or it is transferredto an external display device or memory device (1004). The result of thesensitivity distribution calculation at the first image obtainment ismemorized in a specified address location of a memory in the signalprocessing unit.

Next, the second image is obtained. When the second image is obtained,composing processing 1003 to remove an aliasing artifact is performed byusing the sensitivity distribution data calculated in the first imageobtainment stored in memory after the signal obtainment. After that, aslong as the disposition of signal receiving coil does not change,calculation of the sensitivity distribution is not performed and acomposing process is performed using the sensitivity distribution datain memory. Since the sensitivity distribution does not change when thedisposition of receiving coil does not change, the result of the firstsensitivity distribution calculation, as described above, can be used.Thus, even when the imaging interval (interval between an imageobtainment and the next) is shortened in dynamic imaging, an operationfor imaging can be performed quickly, and the ability to generate adisplay in real time (time resolution) can be improved. For example,when a fast sequence, such as EPI, is adopted as an imaging sequence, itis important to make the operation fast for performing effectivecontinuous fast imaging, and the condition of the object can bemonitored in real time by applying the embodiment shown in FIG. 10.

Next, an MRI apparatus that employs the EPI sequence suitable for theMRI apparatus described above will be explained. In this embodiment, thecomposition of the apparatus is the same as the one shown in FIG. 1. TheMRI apparatus in this embodiment employs the EPI sequence forcontrolling the collection of measured data as a control sequence incontrol unit 111 so as to have a different density at each region of themeasuring space (k space). FIG. 11 shows one embodiment of such an EPIsequence, and an EPI sequence of the spin echo type is shown there. Thatis, the RF pulse 1101 for exciting nuclear spin of organization of theobject and a slice selective gradient magnetic field pulse Gs 1103 areapplied, and after the time period TE/2, the RF pulse 1102 for invertingthe transverse magnetization generated from the first RF pulse 101 isapplied, together with slice selective gradient magnetic field pulse Gs1104; and, after that, the echo signal 1106 is measured whilecontinuously applying the readout gradient magnetic field 1105 thatalternates in polarity. In this procedure, phase encoded gradientmagnetic fields Ge 1107 and 1108 for the phase encoding echo signal areapplied.

In the ordinary EPI sequence, for example, if the matrix size of the kspace is 128×128, the readout gradient magnetic field is inverted 128times, and 128 echo signals are measured. The phase encoded gradientmagnetic field pulse is applied with 128 pulses of the same size otherthan the first offset pulse. But, in the EPI sequence of the presentembodiment, for example, the echo signal of from the phase encoding 0 tothat of a predetermined phase encoding is subjected to a phase encodedgradient magnetic field pulse 1107 of such a size that the number ofphase encodings increments one by one. The echo signal measured afterthat is subjected to the phase encoded gradient magnetic field pulse1108 of such size that it increments by four at each step.

In the example shown in the figure, the case of measuring eight echosignals is shown for simple explanation. Until the fifth signalmeasurement, a phase encoded gradient magnetic field pulse 1107 ofordinary size is applied. From the sixth to the eighth signalmeasurement, a phase encoded gradient magnetic field pulse 1108 having asize four times as large as the ordinary size is applied. Signals fromthe first to the fifth measurement shown in FIG. 4 correspond to region1(402) in the k space, and signals from the sixth to the eighthmeasurement correspond to region 2(403). Then, whereas signals from thesixth to the eighth measurement have hitherto needed time for measuring12 (=3×4) signals, the measuring time for nine (=12−3) signals can beused to shorten this sequence. Regarding the k trajectory, measurementcan be performed thickly in region 1 and can be performed roughly inregion 2 while performing said sequence. In executing the EPI sequenceshown in FIG. 11, only the top or bottom part of region 2 shown in FIG.4 is measured. In this case, the part that is not measured in region 2is estimated by jointly using measured data, a known half Fourier methodand a signal estimating processing method utilizing complex conjugation.

The present invention can be applied to not only a spin echo type EPIsequence as described above, but it can also be applied to a gradientecho type EPI, and to a one shot type EPI collecting necessary measureddata with one excitation or to a multishot type (division type) EPI.

The MRI apparatus based on the second embodiment of the presentinvention is suitable for use as a sequence of parallel MRI using saidmultiple coil. Also, in the case of applying said EPI sequence to aparallel MRI, the process performed in signal processing unit is thesame as that in the case of a GrE sequence. In other words, as shown inFIG. 5, by performing sensitivity distribution calculation 503 at eachsmall type RF coil in the multiple coil, using data measured by said EPIsequence, together with composing process 504 using the calculatedsensitivity distribution, an image from which an aliasing artifact isremoved can be obtained.

And, in the case of dynamic imaging, a sensitivity distributioncalculation is performed only in the reconstruction of the first image,as shown in FIG. 10, and, as for images after that, the process fromsignal obtainment to image display and transfer is performedcontinuously using the same sensitivity distribution. In this case, asdescribed above, the signal obtaining time of the EPI sequence of thepresent invention is shortened compared with an ordinary EPI sequence.Besides, the sensitivity distribution calculation of each signalobtainment does not need to be performed apart from measurement forimaging, or the sensitivity distribution calculation. And so, extremelyfast continuous imaging can be performed.

The above explanation is directed to various embodiments of the MRIapparatus of the present invention. However, it is possible to modifythe present invention in various ways, not limited by these embodiments.For example, though a GrE sequence and an EPI sequence have beenillustrated as a pulse sequence for performing parallel MRI in theexplanation set forth above, the present invention also can be appliedto a known sequence, such as a FSE (fast spin echo sequence), a SE (spinecho sequence), a Burst sequence, and a spiral sequence. In addition, itcan be also expanded to three dimensional imaging.

FIG. 12 is a view showing the k trajectory 1200 in the case of a spiralsequence. In this case, circular region 1 at the center of the kspace(1202) is a region where signals are arranged thickly, and inregion 2 surrounding it, signals are arranged roughly. To calculate thesensitivity distribution from measured data of the spiral sequence, atwo-dimensional filter having a filter profile 1301, 1302, as shown inFIGS. 13(a), 13(b) and 13(c), is used. As this two-dimensional filter, atwo-dimensional Gaussian filter, a two-dimensional hanning filter, atwo-dimensional hamming filter or the like are used. By using thesensitivity distribution at each small type coil calculated frommeasured data of the region 1(1202), an image in which an aliasingartifact is removed is obtained. This is the same as the case in anothersequence.

According to the MRI apparatus of the present invention, in performingparallel MRI using a multiple coil, imaging is performed whileshortening the measuring time by thinning out one part of a region inthe k space, and the sensitivity distribution is calculated using dataof the region where the measured data is thick to compose signals.Therefore, the image quality is not deteriorated in an imaging in whichreal-time imaging is demanded. Especially, by obtaining data thickly ofthe low frequency component in the k space, an image having a high S/Nratio and a high diagnosis value can be obtained.

Also, since there is no need to measure the sensitivity distribution-ofthe RF coil before imaging, the total measure time is not extended.Therefore, the effect of imaging in a short time, which is a maincharacteristic of parallel MRI techniques, can be achieved.

What is claimed is:
 1. A magnetic resonance imaging method of imaging anobject to be examined at high speed using a magnetic resonancephenomenon, comprising the steps of: (a) applying an RF pulse, a sliceencoded gradient magnetic field, a phase encoded gradient magneticfield, and a readout gradient magnetic field to the object to beexamined, which has been placed in a uniform static magnetic field, inaccordance with a predetermined pulse sequence, and executing this pulsesequence repeatedly; (b) detecting NMR signals generated from the objectto be examined by executing said detecting step at each of a pluralityof small type RF coils which form a multiple coil, and memorizing theresulting NMR signals in a memory corresponding to a k space for each RFcoil separately; and (c) performing image composing processing using asensitivity distribution of an RF coil and measured data memorized insaid memory, wherein in a step (a) a step change of an applicationamount of said phase encoded gradient magnetic field in measuring a lowspatial frequency region of the k space is made to be smaller than astep change in measuring a high spatial frequency region therein, insaid pulse sequence executed repeatedly, between step (b) and step (c)there is further step (d) performing a process of calculation of asensitivity distribution of each RF coil using data of the low spatialfrequency region of the k space, and in step (c) using the sensitivitydistribution of an RF coil calculated in step (d).
 2. The magneticresonance imaging method defined in claim 1, wherein a low path filteris used for extracting said low spatial frequency region alone of said kspace in the process of calculating the sensitivity distribution at eachcoil to make the data of said high spatial frequency region zero and tomake the connection between an abstracted region and a zero regionthereof smooth.
 3. The magnetic resonance imaging apparatus defined inclaim 3, wherein the region of k space, except where abstracted data ofsaid low spatial frequency region alone is memorized, is filled withzeros with the same density as the low spatial frequency region, and thek space comprised of this low region data and the zero data of the otherregion is subjected to Fourier transformation to calculate thesensitivity distribution of an RF coil.
 4. The magnetic resonanceimaging method as defined in claim 1, wherein in step (d) thecalculation of the sensitivity distribution of each RF coil uses dataonly of the low spatial frequency region of the k space.
 5. The magneticresonance imaging method as defined in claim 1, wherein in step (a) thestep change of the application amount of said phase encoded gradientmagnetic field in measuring the low spatial frequency region of the kspace is made to be one step amount of the phase encoded gradientmagnetic field, in step (b) detecting every NMR signal corresponding tothe low spatial region of the k space, in step (d) the calculation ofthe sensitivity distribution of each RF coil uses all data of the lowspatial frequency region of the k space.
 6. The magnetic resonanceimaging method as defined in claim 1, wherein in step (b) memorizing NMRsignals for an image and NMR signals for the sensitivity distributionact effected in separate memory us corresponding to the k spacerespectively, in step (d) the calculation of the sensitivitydistribution of each RF coil uses at least one part of data on the lowspatial frequency region of the k space memorized in the memory of theNMR signals for the sensitivity distribution.
 7. A magnetic resonanceimaging method of imaging an object to be examined at high speed using amultiple coil, comprising the steps of: (a) preparing a memory having apredetermined matrix size in which to memorize NMR signals detected fromthe object to be examined; (b) executing a pulse sequence for NMRimaging of the object to be examined, which has been placed in a uniformstatic magnetic field; (c) memorizing NMR signals obtained by executingsaid pulse sequence into said memory; and (d) composing an image byusing a sensitivity distribution of a plural number of small type signalreceiving coils forming said multiple coil and NMR signals memorized insaid memory; wherein between step (c) and (d) there is further step (e)calculating the sensitivity distribution of the small type signalreceiving coils by using one part of said NMR signals for imaging, whichhave been memorized in said memory, and in step (d) using thesensitivity distribution of the small type signal receiving coilscalculated in step (e).
 8. A magnetic resonance imaging method ofimaging an object to be examined using a multiple coil withoutgenerating an aliasing artifact, comprising the steps of: (a) preparinga memory corresponding to k space having a predetermined matrix size inwhich to memorize NMR signals detected from the object to be examined,the number of NMR signals corresponding to the number of small typesignal receiving coils which form said multiple coil; (b) executing apulse sequence for NMR imaging of the object to be examined, which hasbeen placed in a uniform static magnetic field; (c) memorizing themeasured data for imaging obtained by executing said pulse sequence intosaid memory corresponding to a small type coil; and (d) composing animage of the fields of view of the multiple coil by using a sensitivitydistribution of a small type signal receiving coil and measured datamemorized in said memory; wherein in step (b) data in a low spatialfrequency region is measured finely in the k space, and data in a highspatial frequency region is measured roughly, when said pulse sequenceis executed, between step (c) and step (d) there is further step (e)calculating the sensitivity distribution of the plural number of smalltype signal receiving coils forming said multiple coil by using at leastone part of the data measured in the low spatial frequency region of thek space and memorized in said memory, and in step (d) using thesensitivity distribution of the small type signal receiving coilcalculated in step (e).
 9. The magnetic resonance imaging method asdefined in claim 8, wherein in step (b) the data in the low spatialfrequency region in the phase encoded direction is measured finely inthe k space, and the data in the high spatial frequency region in thephase encoded direction is measured roughly, when said pulse sequence isexecuted.
 10. A magnetic resonance imaging method continuously imagingan object to be examined using a multiple coil, comprising the steps of:(a) preparing a first memory corresponding to a k space having apredetermined matrix size in which to memorize NMR signals detected fromthe object to be examined; (b) executing a pulse sequence for imaging afirst NMR image of the object to be examined, which has been placed in auniform static magnetic field; (c) memorizing NMR signals obtained byexecuting said pulse sequence into said first memory; (d) composing animage by using a sensitivity distribution of a plural number of smalltype signal receiving coils which form said multiple coil and the NMRsignals memorized in said first memory; and (e) executing a pulsesequence for imaging after a second imaging of the object to be examinedand composing an image by applying the sensitivity distribution of thesmall type signal receiving coils to the obtained NMR signal: wherein instep (b) data in a low spatial frequency region is measured finely inthe k space, and data in a high spatial frequency region being measuredroughly, when said pulse sequence is executed, between step (c) and step(d) there is further step (f) calculating the sensitivity distributionof the plural number of small type signal receiving coils by using thedata measured in the low spatial frequency region of the k space whichis memorizing into a second memory, and in step (e) using thesensitivity distribution of the small type signal receiving coilsmemorized in the second memory in step (f).
 11. A magnetic resonanceimaging apparatus comprising: a magnet for generating a uniform staticmagnetic field within a space accommodating an object to be examined; amultiple coil composed of a plural number of small type coils fordetecting NMR signals generated from said object to be examined, saidplural number of small type coil being arrayed so as to overlap in partwith adjacent coils; means for applying a high frequency magnetic field,a slice encoded gradient magnetic field, a phase encoded gradientmagnetic field and a readout gradient magnetic field to image saidobject to be examined, where the phase encoded direction is directed inthe direction of arrangement of said multiple coil; means forcontrolling said magnetic field applying means, including means formodifying a step change in the application amount of said phase encodedgradient magnetic field in its high region relative to that in its lowregion; measured data memorizing means for memorizing an NMR signaldetected in said multiple coil corresponding to each small type coil;means for calculating the sensitivity distribution of each small typecoil by using data of the low region in said phase encoded direction forevery NMR signal detected by each small coil; and means for composing animage from said sensitivity distribution and data memorized in saidmeasured data memorizing means.
 12. The magnetic resonance imagingapparatus defined in claim 11, wherein said measured data memorizingmeans has a predetermined matrix memorizing arrangement, and saidcontrolling means includes means for covering a measured data memorizingaddress location where measured data falls out in high region in thephase encoded direction with data calculated from actual measured dataand said sensitivity distribution.
 13. The magnetic resonance imagingapparatus defined in claim 11, wherein said means for calculating thesensitivity distribution of said small type coils includes filteringmeans for abstracting a low region in the phase encoded direction ofdata memorized in said measured data memorizing means and means forperforming two-dimensional Fourier transformation on composed datacomprised of low region data abstracted and zero data filled in theregion except for the low region.
 14. The magnetic resonance imagingapparatus defined in claim 13, wherein said filtering means comprises afilter having a filter profile for smoothing the connection of databetween the low region abstracted in the direction of phase encoding andthe high region filled with zeros.
 15. A magnetic resonance imagingapparatus, comprising: a magnet for generating a uniform static magneticfield within a space accommodating an object to be examined; a multiplecoil composed of a plural number of small type coils for detecting NMRsignals generated from said object to be examined, said plural number ofsmall type coils being arrayed so as to overlap in part with adjacentcoils; means for applying a high frequency magnetic field, a sliceencoded gradient magnetic field, a phase encoded gradient magnetic fieldand a readout gradient magnetic field in accordance with a predeterminedpulse sequence to image said object to be examined; a k space formemorizing NMR signals detected by said multiple coil corresponding toeach small type coil; means for controlling said magnetic field applyingmeans, including means for modifying a step change in the high regionand in the low region of said k space memorizing measured data; meansfor calculating the sensitivity distribution of each small type coil byusing data of the low region in said k space in every NMR signaldetected by a small type coil; and means for composing an image fromsaid sensitivity distribution and the data memorized in said memorizingmeans.
 16. A magnetic resonance imaging apparatus, comprising: means forexecuting a second measurement repeatedly after a first measurement forimaging a predetermined slice of an object to be examined in an imagingunit; and means for composing an image of all fields of view of amultiple coil by using a sensitivity distribution of the multiple coil,wherein said first measurement measures data in a low spatial frequencyregion finely in a k space and data in a high spatial frequency regionroughly, said second measurement measures the data in all regions of thek space roughly, the apparatus includes means for calculating thesensitivity distribution of said multiple coil by using at least onepart of the measured data in the low spatial frequency region of the kspace acquired in said first measurement which is memorized in a memory,such said image composing means uses said memorized sensitivitydistribution, the apparatus further includes means for applying saidmemorized sensitivity distribution without renewal to the imagecomposition based on measured data after calculating the sensitivitydistribution.
 17. A magnetic resonance imaging apparatus, comprising: amagnet for generating a uniform static magnetic field within a spaceaccommodating an object to be examined; a multiple coil composed of aplural number of small type coils for detecting NMR signals generatedfrom said object to be examined; means for applying a high frequencymagnetic field, a slice encoded gradient magnetic field, a phase encodedgradient magnetic field and a readout gradient magnetic field to imagesaid object to be examined; means for controlling said magnetic filedapplying means, including means for modifying a step change in anapplication amount of said phase encoded gradient magnetic field inmeasuring a high spatial frequency region of a k space relative to thatin a low spatial frequency region; measured data memorizing means formemorizing an NMR signal detected in the multiple coil corresponding toeach small type coil; means for calculating a sensitivity distributionof each small type coil by using data of the low spatial frequencyregion of the k space for each small coil; and means for composing animage from the sensitivity distribution and data memorized in saidmeasured data memorizing means.
 18. A magnetic resonance imagingapparatus, comprising: a magnet for generating a uniform static magneticfield within a space accommodating an object to be examined; a multiplecoil composed of a plural number of small type coils for detecting NMRsignals generated from said object to be examined; means for executing apulse sequence for NMR imaging of said object to be examined; means formeasuring data in a low spatial frequency region around center portionfinely in a k space, and data in a high spatial frequency region awayfrom the center portion roughly, when said pulse sequence is executed;measured data memorizing means for memorizing an NMR signal detected insaid multiple coil corresponding to each small type coil; means forcalculating a sensitivity distribution of each small type coil by usingthe data of at least the low spatial frequency region for each smallcoil; and means for composing an image from said sensitivitydistribution and data memorized in said measured data memorizing means.19. The magnetic resonance imaging apparatus as defined in claim 18,wherein said data measurement means measures the data along a spiraltrajectory through the k space.